The present invention relates to a continuous process for preparing aromatic di- and polyamines by catalytic hydrogenation of the corresponding di- and polynitro compounds.
A large number of processes for preparing aromatic amines by catalytic hydrogenation of nitro compounds is known. See, for example, U.S. Pat. Nos. 3,546,296; 3,781,373; 3,761,521; 3,895,065; 4,288,640; DE 2,135,154; U.S. Pat. No. 3,882,048; GB 1,490,313; GB 1,017,646; U.S. Pat. Nos. 3,356,728; 3,356,729; 3,431,085; 3,194,839 and GB 768,111.
A considerable amount of heat is released when aromatic polynitro compounds react with hydrogen. Attempts to make use of the heat of hydrogenation released during the preparation of aromatic polyamines by hydrogenation of the corresponding polynitro compound have been made. For example, the warm cooling water can be used to heat rooms or to warm product streams or even to evaporate low boiling solvents.
Generally, the large-scale hydrogenation of aromatic polynitro compounds is performed at the lowest possible temperature to reduce the risk of uncontrolled side reactions when hydrogenating aromatic polynitro compounds at elevated temperatures. Side reactions may lead to the formation of undesired by-products and thus to reduction in yield. Ring-hydrogenation, hydrogenolytic decomposition or the formation of macromolecular, tar-like products are examples of undesirable side reactions. Explosive side reactions may also take place. Such side reactions are caused by the strongly exothermic reaction of nitro-groups and their high rates of reaction at elevated temperatures.
To avoid these undesired side reactions as much as possible, the large-scale hydrogenation of aromatic polynitro compounds has generally been performed at temperatures such that the production of steam with an excess pressure of more than 2 bar would not take place.
Canadian Patent 1,211,757 describes a process for preparing aromatic dinmines by catalytic hydrogenation of the corresponding dinitro compounds with the simultaneous production of steam with an excess pressure of &gt;1 bar. A bubble column provided with filled pipes is used as the reactor. The reactor is cooled by means of water which is converted into steam in the field pipes. A reaction suspension made up of an aromatic dinitro compound, the corresponding diamine, a hydrogenation catalyst, a saturated, aliphatic alcohol with 1 to 6 carbon atoms as solvent and water, is fed to the reactor with the hydrogen. The amount of reaction suspension supplied to the bubble column, and the pressure, temperature and amount of cooling water, are chosen so that the reaction temperature in the bubble column is between 140.degree. and 250.degree. C.
The disadvantage of the process described in Canadian Patent 1,211,757 is the use of a solvent. Although the solvent moderates the known problems of hydrogenation of polynitro compounds at elevated temperatures, it is not completely inert under the hydrogenation conditions. The solvent reacts to form undesired side products and reduces the yield of amine. Upon completion of the reaction, the solvent must be separated from the aromatic diamine and optionally worked up. Such treatments increase the cost of the process.
Attempts have been made to transfer the essential features of the process described in Canadian Patent 1,211,757 to a solvent-free catalytic hydrogenation of aromatic nitro compounds. EP 263,935 discloses a process in which stirred reactors are used to perform exothermic reactions. These reactors are cooled by means of water which is convened into steam with more than 1 bar excess pressure in field pipes. The ratio of cooling surface in the boiling tubes to the volume of the reaction space is 40 to 400 m.sup.2 /m.sup.3. This high ratio has proven to be particularly effective for removal of the heat of reaction which is released. However, the process disclosed in EP 263,935 can only be Used to a limited extent for the catalytic hydrogenation of polynitro aromatic compounds because complete phase-mixing is not guaranteed. Due to inhomogeneities, intense and uncontrollable side reactions take place. These side reactions result in reduction in yield and coating of the cooling surfaces with resin-like compounds and/or a proportion of the catalyst.